control of pre-rift lithospheric structure on the ...continental rifts and conjugate passive margins...

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Control of pre-rift lithospheric structure on the architecture and evolution of

continental rifts: insights from the Main Ethiopian Rift, East Africa

Giacomo Corti1, Paola Molin2, Andrea Sembroni2, Ian D. Bastow3, Derek Keir4,5

1. Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse, Via G. La Pira, 4, 50121

Florence, Italy

2. Università degli Studi di Roma Tre, Dipartimento di Scienze, Largo San Leonardo Murialdo, 1, 00146

Roma, Italy

3. Department of Earth Science and Engineering, Imperial College London, London, SW7 2AZ, U.K.

4. Ocean and Earth Science, University of Southampton, Southampton, SO14 3ZH, U.K.

5. Dipartimento di Scienze della Terra, Università degli Studi di Firenze, via G. La Pira 4, 50121

Florence, Italy

Abstract

We investigate the along-axis variations in architecture, segmentation and evolution of the

Main Ethiopian Rift (MER), East Africa, and relate these characteristics to the regional

geology, lithospheric structure and surface processes. We first illustrate significant along-axis

variations in basin architecture through analysis of simplified geological cross-sections in

different rift sectors. We then integrate this information with a new analysis of Ethiopian

topography and hydrography to illustrate how rift architecture (basin symmetry/asymmetry)

is reflected in the margin topography and has been likely amplified by a positive feedback

between tectonics (flexural uplift) and surface processes (fluvial erosion, unloading). This

analysis shows that ~70% of the 500 km-long MER is asymmetric, with most of the

asymmetric rift sectors being characterized by a master fault system on the eastern margin.

We finally relate rift architecture and segmentation to the regional geology and geophysical

constraints on the lithosphere. We provide strong evidence that rift architecture is controlled

by the contrasting nature of the lithosphere beneath the homogeneous, strong Somalian

Plateau and the weaker, more heterogeneous Ethiopian Plateau, differences originating from

the presence of pre-rift zones of weakness on the Ethiopian Plateau and likely amplified by

surface processes. The data provided by this integrated analysis suggest that asymmetric rifts

may directly progress to focused axial tectonic-magmatic activity, without transitioning into a

symmetric rifting stage. These observations have important implications for the asymmetry of

continental rifts and conjugate passive margins worldwide.


Key points

Structural and geomorphological analysis constrain variations in architecture,

segmentation and evolution of the Main Ethiopian Rift

More than half of the rift is asymmetric, with architecture and segmentation primarily

controlled by the pre-rift lithospheric structure

Asymmetric rifts directly progress to focused axial tectonic-magmatic activity,

without transitioning into a symmetric rifting stage

1. Introduction

Continental rifts often have considerable along-strike variations in architecture and

magmatism. Variations in fault pattern and evolution may be the result of variations in pre-

existing lithospheric rheology, rift width, as well as extension rate and kinematics (e.g.,

Ebinger, 2005; Ziegler and Cloetingh, 2004; Corti, 2012; Brune, 2016). Along-axis

differences in magmatism are commonly explained by processes such as variable mantle

potential temperature, heterogeneous anomalous volatile content in the asthenosphere,

variable extension rate or processes such as melt focusing at steep gradients of the

lithosphere-asthenosphere boundary (e.g., Keir et al., 2015 and references therein). Among

these parameters, pre-existing plate structure is expected to play a major role in controlling

continental rift architecture, because variations in crustal and lithospheric vertical layering

and strength and/or the presence of inherited heterogeneities may significantly influence the

style and distribution of deformation at both local and regional scale (e.g., Ziegler and

Cloetingh, 2004; Sokoutis et al., 2007; Corti, 2012; Brune, 2016). Continental rifting results

from the application of extensional stresses to a pre-deformed, anisotropic lithosphere and,

consequently, rift structures are not randomly distributed. They instead tend to follow the

trend of pre-existing weaknesses (such as ancient orogenic belts), avoiding stronger regions

such as cratons (e.g., Dunbar and Sawyer, 1989; Versfelt and Rosendahl, 1989; Vauchez et

al., 1997; Tommasi and Vauchez, 2001; Ziegler and Cloetingh, 2004; Buiter and Torsvik,

2014). In these conditions, variations in the mechanical properties of the rifting plate (e.g.,

Scholz and Contreras, 1998) and/or the presence of the pervasive and/or discrete fabrics (e.g.,

Versfelt and Rosendahl, 1989; Ring, 1994) may exert a primary control on the extension-

related pattern of faulting at both regional and local scale (e.g., Ziegler and Cloetingh, 2004;

Corti, 2012; Brune, 2016). Recent studies support the strong influence played by along-axis


variations in basement fabrics on rift segmentation, inducing significant variations in the

characteristics of the rift margins (e.g., vertical throw on major boundary faults) and the

symmetry or asymmetry of the rift basins (see for instance examples from Lake Malawi in

Lao-Davila et al., 2015 or from the Upper Rhine Graben in Grimmer et al., 2017).

Differences in rift symmetry/asymmetry are also typically interpreted to be time-dependent:

observations from the East African Rift System suggest a progression from asymmetric to

symmetric rifting, eventually followed by axial focusing of the volcanism and faulting (e.g.,

WoldeGabriel et al., 1990; Hayward and Ebinger, 1996). Moreover, numerical modeling

indicates that unloading related to fluvial incision enhances the flexural uplift at major rift

escarpments, generating a positive feedback between tectonics and surface processes (Tucker

and Slingerland, 1994; Kooi and Beaumont, 1994; Petit et al., 2007). Therefore the dynamic

coupling between flexural uplift and fluvial erosion may significantly influence the

symmetric or asymmetric structure of the rift.

In this contribution, we integrate detailed geological-structural and geomorphological

analysis to investigate the along-axis variations in architecture, segmentation and evolution of

the different rift sectors of the Main Ethiopian Rift (MER), East Africa, and relate these

characteristics to the lithospheric structure and surface processes. The MER is a classical

example of continental rifting developed within a highly anisotropic lithosphere that has

experienced several tectonic events over the last one billion years of geological history and

tectonics (e.g., Abbate et al., 2015). It is an ideal study locale for the analysis of rift structure

in conjunction with surface processes since the expression of different stages in the

evolutionary rift sequence, from initial rifting in the south to incipient break-up in the north,

are all subaerially exposed and rigorously studied at depth using geophysical techniques (e.g.,

Corti, 2009; Keir et al., 2013; Ebinger et al., 2017). To investigate the variations in the

characteristics of rifting in the MER, we first illustrate simplified geological cross-sections in

different portions of the rift, which show significant along-axis variations in the distribution

of extension-related deformation and basin architecture. We then use new analysis of the

topography and hydrography of the study area as an independent analysis of basin

architecture, and to further investigate quantitatively the erosion-tectonics feedback and the

response of surface processes to the variable rift architecture along the MER. We relate rift

architecture and segmentation to the pre-rift rheology of the continental lithosphere and

demonstrate a significant correlation between asymmetry (presence of half graben) of the

MER and locally weak lithosphere on the side of the rift lacking a main border fault. We

interpret an important influence exerted by the contrasting (pre-rift) nature of the lithosphere


beneath the Ethiopian and Somalian plateaus surrounding the rift valley. We finally show

how the data provided by this integrated analysis suggest that rift sectors characterised by an

asymmetric architecture can directly progress to focused axial tectono-magmatic activity,

without transitioning into a symmetric rifting stage, therefore challenging previous views of

evolution of rifting in the East African Rift System (e.g., WoldeGabriel et al., 1990; Hayward

and Ebinger, 1996). These observations have important implications for the asymmetry of

continental rifts and conjugate passive margins worldwide, which has often been explained in

the context of low-angle detachment faults and models involving a lithospheric-scale simple

shear deformation (e.g., Wernicke, 1985).

2. Geological setting

The MER, at the northern end of the East African Rift System, accommodates 4-6 mm/yr,

N100°E oriented extension between the major Nubia and Somalia Plates (e.g., Saria et al.,

2014). This rift is traditionally subdivided into three main sectors, the Southern, Central, and

Northern MER (SMER, CMER and NMER, respectively), each differing in terms of fault

timing and patterns, and lithospheric characteristics (Fig. 1; e.g., Mohr, 1983; WoldeGabriel

et al., 1990; Hayward and Ebinger, 1996; Bonini et al., 2005; Corti, 2009).

The MER sectors are affected by two distinct systems of normal faults: (1) the border faults

and (2) a set of faults affecting the rift floor, often referred to as the Wonji Fault Belt (WFB:

e.g., Mohr, 1962; Boccaletti et al., 1998). The border faults are long (≥50 km), large-offset

(typically >500m) structures that give rise to the major escarpments separating the rift floor

from the surrounding plateaus. These faults have been activated diachronously in different

rift segments during the last 10-12 Ma (e.g., Balestrieri et al., 2016). Geological data and

analysis of the historical and instrumental seismicity suggest that the boundary faults are

largely inactive (e.g., Wolfenden et al., 2004; Casey et al., 2006; Keir et al., 2006).

Conversely, morphotectonic and geological analysis and Global Positioning System (GPS)

data indicate that these faults still accommodate significant extension in the CMER and

SMER (Gouin, 1979; Keir et al., 2006; Pizzi et al., 2006; Agostini et al., 2011a; Kogan et al.,

2012; Molin and Corti, 2015).

The WFB is an axial tectono-volcanic system characterized by relatively short (typically

<20km-long), closely spaced, active faults that exhibit minor vertical throw (typically

<100m) with associated extension fractures and grabens (e.g., Boccaletti et al., 1998;

Acocella et al., 2003). These faults, which are intimately associated with intense Quaternary-

Recent magmatism of the rift floor, developed in the last 2 Ma (e.g. Boccaletti et al., 1998;


Ebinger and Casey, 2001). The WFB is well expressed in the NMER, where its structures

form clearly-defined right-stepping en-echelon segments obliquely cutting the rift floor

(Ebinger and Casey, 2001). The abundance of Wonji faults decreases southwards: these faults

are in an incipient stage of development in the CMER, but practically negligible in the SMER

(Fig. 1; e.g., Agostini et al., 2011b).

Along the MER, Quaternary-Recent volcanism is dominated by rhyolites, ignimbrites,

pyroclastic deposits, and subordinate basalts (e.g., WoldeGabriel et al., 1990). In the NMER,

this activity is strongly focused within the axial WFB (e.g., Ebinger and Casey, 2001).

Geophysical data indicate that extension is currently accommodated via a combination of

magma intrusion and normal faulting (e.g., Mackenzie et al., 2005; Keranen et al., 2004; Keir

et al., 2006), with magma intrusion modifying the composition and thermal structure of the

crust and lithosphere beneath the WFB (e.g. Beutel et al., 2010; Daniels et al., 2014).

Magmatic modification of the crust/mantle lithosphere and Quaternary-Recent volcanism

decreases southwards. In the SMER, consistent with a limited volcano-tectonic activity

within the rift, the albeit-sparse geophysical data indicate the absence of significant magmatic

modification of the crust/lithosphere (Dugda et al., 2005; Daly et al., 2008).

Overall, these along-axis variations of the distribution and characteristics of the tectono-

magmatic activity have been interpreted to reflect a transition from initial rifting in the

SMER, with marginal deformation and fault-dominated rift morphology, to advanced rifting

stages in the NMER, where prominent axial intrusion, dyking and associated normal faulting

testify a phase of magma-assisted rifting during the late stages of continental rifting (e.g.,

Kendall et al., 2005).

Geophysical studies in the MER suggest that rift location and initial evolution has been

controlled by a NE-SW- to N-S-trending lithospheric-scale pre-existing heterogeneity (e.g.,

Gashawbeza et al., 2004; Bastow et al., 2005, 2008; Keranen and Klemperer, 2008; Keranen

et al., 2009; Cornwell et al., 2010), which largely controlled the trend of the Cenozoic rift

faulting (e.g., Mohr, 1962; Kazmin et al., 1980). This pre-existing heterogeneity

corresponded to a suture zone which separated two distinct Proterozoic basement terranes

beneath the Ethiopian and Somalian plateaus (e.g., Vail, 1983; Berhe, 1990; Stern et al.,

1990; Stern, 2002; Abdelsalam and Stern, 1996). Extension occurred at approximately the

same time as, and after, hotspot-related flood basalt volcanism (Wolfenden et al., 2004),

possibly connected to the African Superplume found in the lower mantle beneath southern

Africa (e.g., Ritsema et al., 1999). The reactivation of the pre-existing heterogeneity is

interpreted to have occurred at the eastern edge of the upwelling mantle not above its centre


(e.g., Bastow et al., 2008), as documented in other flood basalt provinces (Courtillot et al.,

1999) and suggested by theoretical modeling (Tommasi and Vauchez, 2001).

Several tectonic events affected Ethiopia prior to the Cenozoic rifting, from collision during

the Precambrian to Mesozoic extension (Abbate et al., 2015 and references therein). This

long pre-Cenozoic history of tectonic events created pre-existing heterogeneities with

variable orientation that have controlled the development of the East African Rift System in

Ethiopia (e.g., Korme et al., 2004). At a regional scale, for instance, large-scale E-W or

WNW-ESE heterogeneities have controlled the development of major transversal volcano-

tectonic lineaments (such as the Yerer-Tullu Wellel Volcanic Lineament: YTVL and the

Goba-Bonga; Fig. 1) affecting the rift and the surrounding plateaus (e.g., Abebe Adhana,

2014 and references therein) and marking the transition between the different MER sectors.

At a more local scale, pre-existing basement fabrics may have controlled fault and volcanic

edifice geometries (e.g., Corti, 2009 and references therein).

3. Methods and results

3.1 Structural cross-sections in the different MER segments

Simplified geological profiles across different sectors of the MER are illustrated in Fig. 2. In

these, the subsurface geology and the resulting basin architecture are derived from published

cross-sections, extrapolation of surface geology or available subsurface data (see list of

references in the caption of Fig. 2). Faults are differentiated based on their exposed minimum

vertical displacement (e.g., Lao-Davila et al., 2015), a value that underestimates the actual

fault motion since other processes (e.g., erosion, deposition) likely altered the topography of

the plateaus and the rift floor (see below). Henceforth, we use the term 'rift symmetry' when

the cumulative exposed minimum vertical displacement on major boundary faults is similar

on both rift margins, and the term 'rift asymmetry' when this value is significantly lower on

one margin with respect to the other (e.g., Lao-Davila et al., 2015). This classification is

supported by basin architecture derived from the above-mentioned analysis.

Profile 1 crosses the northern portion of the NMER and shows an approximately

symmetric rift structure, with major boundary fault systems on both western and

eastern margins (Ankober and Arboye, respectively; Fig. 1); the recent tectono-

magmatic activity of the WFB is well developed at the rift axis in this area (Fantale-

Dofan magmatic segment; e.g., Ebinger and Casey, 2001). The boundary faults are

generally interpreted to have been deactivated during the Pleistocene (Wolfenden et


al., 2004; Casey et al., 2006), although the high levels of seismicity recorded in the

period 2002-2003 indicates ongoing fault slip along the Ankober border fault system

(Keir et al., 2006). However, most active tectonics in the area is believed to be

accommodated via a combination of intrusion, dyking and normal faulting within the

WFB (Keir et al., 2006). This portion of the rift passes into a more asymmetric rift

structure northwards into Afar, with a major escarpment on the western side (Ankober

fault system; e.g., Wolfenden et al., 2004).

Similarly, the southern portion of the NMER (Profile 2) displays a marked

asymmetry, with the large offset main boundary fault system on the eastern side (Sire

fault system) and the northwestern margin characterised by a faulted flexure (e.g.,

Wolfenden et al., 2004). The axial portion of the rift is affected by recent tectonic and

magmatic activity associated with the WFB (Boseti magmatic segment), which

accommodates the largest portion of current plate motion (e.g., Kogan et al., 2012).

Profile 3 illustrates the across-axis structure of the northern portion of the CMER,

which is characterized by a major boundary fault system on the western margin

forming the Guraghe escarpment. The opposite side of the rift is marked by the Asela-

Sire fault system, whose morphological expression may be at least in part obscured by

the prominent Pliocene-Quaternary volcanism of large volcanoes (such as Chilalo) at

the rift margin (Fig. 1; e.g., Boccaletti et al., 1999). A general tilting of eastern margin

towards the rift axis is observed (e.g., Pizzi et al., 2006; Agostini et al., 2011a);

although this tilting may be locally controlled by loading from magma intrusion (Corti

et al., 2015), the margin architecture may support a rift asymmetry with master fault

on the western side. The recent tectono-magmatic activity is well developed on the

WFB close to the Asela margin, east of Lake Ziway; Quaternary-Recent magmatic

activity also characterized the western margin on the Silti Debre Zeit alignment,

although not associated with surface faulting (e.g., Agostini et al., 2011a). Both the

boundary fault systems and the WFB accommodate the recent-current deformation in

the area (e.g., Keir et al., 2006; Pizzi et al., 2006; Agostini et al., 2011a; Molin and

Corti, 2015).

Further south, the CMER (Profile 4) presents well-expressed, large boundary faults at

Fonko and Langano (e.g., Agostini et al., 2011a), which give rise to a roughly

symmetric architecture. The WFB volcano-tectonic activity is developed close to the

rift axis, and well expressed by the O'a caldera, now occupied by Lake Shala. Both


the rift margins and the WFB are active in this rift sector, and accommodate the

recent-current deformation in the area (e.g., Keir et al., 2006; Agostini et al., 2011a;

Molin and Corti, 2015).

Profile 5 illustrates the SMER north of Lake Abaya, which is an asymmetric structure

characterized by a major fault escarpment on the eastern side (Agere Selam

escarpment) and a faulted, riftward-dipping monocline at the Soddo margin on the

western side (e.g., Corti et al., 2013; Philippon et al., 2014). This latter margin is

affected by numerous, closely spaced, small normal faults with associated widespread

Pleistocene-Holocene volcanism (Fig. 1). The architecture of faults and the

distribution of volcanic vents resemble the tectono-magmatic architecture of the

WFB, but magma geochemistry is akin to that of the Silti Debre Zeit volcanic belt.

Overall, the current volcano-tectonic activity in the north Abaya region is mostly

accommodated at the western margin of the rift; conversely, there is little evidence for

significant current tectonic activity on the eastern margin (Rooney, 2010; Kogan et

al., 2012; Corti et al., 2013; Philippon et al., 2014).

Further south (Profile 6) rifting becomes more complex: deformation affects a much

wider region and is partitioned between the SMER and the Gofa Basin and Range or

Gofa Province (e.g., Ebinger et al., 2000; Philippon et al., 2014 and references

therein). The SMER itself is subdivided in two different basins, Chamo and Galana,

separated by a prominent basement horst (Amaro Horst). The Galana basin displays a

marked asymmetry with a major boundary fault system on the western side, whereas

the Chamo basin is more symmetric (Ebinger et al., 1993). The minor basins

composing the Gofa Province are generally asymmetric, with a master fault on the

western sides (e.g., Ebinger et al. 2000). Recent volcanism is almost absent in these

basins, and the current tectonic activity is likely accommodated by large boundary

faults of both the SMER (e.g., Kogan et al., 2012; Philippon et al., 2014) and the Gofa

Province (e.g., Ebinger et al., 2000; Philippon et al., 2014).

Overall, analysis of these data indicate that the MER displays significant along-axis

variations in structure and can be subdivided in different portions characterized by symmetric

or asymmetric structure.


3.2 Topography and hydrography patterns

We conduct quantitative analysis of topography and hydrography patterns as an independent

test of rift symmetry. The flexural uplift generates topography and, as a consequence, rivers

incise, further unloading the footwall and enhancing its isostatic uplift (Tucker and

Slingerland, 1994). This positive feedback should increase the possible asymmetry of the rift

structure. The role of erosion in shaping topography is considered of first order importance

since surface processes exert a control on regional scale phenomena (Tucker and Slingerland,

1994 and references therein), and has been widely explored in orogenic belts (Molnar and

England, 1990; Willett, 1999; Willett et al., 2001; Brocklehurst and Whipple, 2002; Burbank,

2002; Stern et al., 2005; Wittman et al., 2007; Champagnac et al., 2009 and references

therein) and passive margins (Pazzaglia and Gardner, 1994; Kooi and Beaumont, 1994;

Tucker and Slingerland, 1994; Anell et al., 2009). However, little progress has been made to

understand such dynamics in the context of extensional tectonics. The works available

concentrate mainly on the mechanical unloading due to extension (Weissel and Kerner, 1989;

Stern and Brink, 1989; Weissel et al., 1995; Poulimenos and Doutsos, 1997; Mueller et al.,

2005; Petit et al., 2007) or on the interaction between surface and large-scale lithosphere

processes (Burov and Cloetingh, 1997; Daradich et al., 2003; Huismans and Beaumont, 2005;

Pik et al., 2008; Mueller et al., 2009).

In the following sections, we analyze the topography and hydrography of the MER to place

fully independent constraints on the symmetry of the rift, and to characterize the major

surface processes and to investigate the possible role of erosion in rift architecture and its

along-axis variations (Fig. 3). We use NASA's Shuttle Radar Topography Mission (SRTM)

digital elevation models, whose ~90 m resolution is suitable for regional analysis.

3.2.1 Swath profiles

To describe and quantify the general topographic trend of the MER and surroundings, we

generate six swath profiles coincident with the structural profiles (Figs. 1, 3). They show the

trend of maximum, minimum and mean elevation in a single plot (Isacks, 1992; Molin et al.,

2004; Ponza et al., 2010). We extract them from the SRTM digital elevation models in GIS

environment, sampling topography every 2 km into observation windows 30 km wide. The

length of each profile is chosen to represent the MER, as in the structural profiles, but also

the surroundings to better show the possible river response to tectonic input. In the swath

profiles, in correspondence with each rift margin, we measured the drop in elevation from the

highlands to the rift floor and the relative topographic gradient. Since the rift floor is


blanketed by volcanic edifices and deposits, the measurements consider just the topographic

expression of the interaction between tectonics, volcanism and surface processes.

Swath profile 1 extends from the Ethiopian Plateau in the NNW to the Somalian Plateau in

the SSE (Figs. 3, 4). In the NNW, the trend of mean topography shows the flexural uplift

affecting the rift shoulder (2.5-3 km of elevation), as documented by Weissel et al. (1995)

and Sembroni et al. (2016b). A similar flexural strain is possibly hidden by the strong fluvial

erosion affecting the SSE sector where the Somalian Plateau is limited to a narrow strip of

land with a mean elevation of ~2700 m (Figs. 3, 4). The MER appears symmetric, limited on

both sides by escarpments with similar height and topographic gradient (Table 1). Profile 1

shows also a large difference (~1000 m) in maximum and minimum topography along the

northwestern rift shoulder, indicating high local relief. Molin and Corti (2015) interpreted this

as typical of a mature stage of rifting when rift margins are highly incised by a well organized

fluvial network composed by concave and steep rivers.

Swath profile 2 is located immediately to the east of Addis Ababa where the roughly E-W

trending transverse structures, namely the YTVL (Abebe et al., 1998), separates the NMER

and CMER (Philippon et al., 2014). This profile shows the rift as an asymmetric structure

(Fig. 4) confirmed also by the large difference in topographic gradient between the western

and eastern margins (Table 1): the mean elevation of the Ethiopian Plateau decreases

progressively from ~3000 m to <1500 m toward the rift floor in a distance of ~60 km;

conversely, the Somalian Plateau is well defined with a maximum almost flat topography

attained at 2500 m. Here, the difference in elevation of about 1000 m between maximum and

minimum topography evidences a strong incision of the inner portion of the Somalian

Plateau. This strongly contrasts with the Ethiopian Plateau, where incision is <500 m.

Swath profile 3 in the CMER shows, on both Ethiopian and Somalian plateaus, a large

difference between maximum and minimum topography, i.e. a local relief of ~1000 m. West

of the CMER, the Ethiopian Plateau is eroded by the tributaries of the Omo River so that its

westernmost portion appears to be separated from the Guraghe structure. East of the CMER,

the highlands are poorly eroded except where the valley of the Wabe River is located (Fig. 3).

The MER shows an asymmetrical configuration with the Guraghe steep escarpment on the

northwestern side (~1500 m in height) and a step-like scarp on the southeastern side where

the highest peak corresponds to the Galama Volcanic Range. Such configuration is well

described by the differences in rift margin elevation drop and topographic gradient values,

being the western margin characterized by much higher values than the eastern one (Table 1).


In Swath profile 4 (southern portion of the CMER; Figs. 3, 4) the mean topography is slightly

above 2000 m west of the Omo River valley, but increases (to >2500 m) towards the rift

margin. Similarly, the Somalian Plateau is ~2500 m except where volcanoes (Mt Chilke, Mt

Kaka, Bale Mts) rise from it. Here the MER presents a nearly symmetrical shape with

escarpments ~1000 m high characterized by a similar elevation drop (~900 m on average;

Table 1). Such a trend is not confirmed by the topography gradient which is slightly higher in

the eastern margin (Table 1). Such discrepancy is related to the different distribution of

extension-related deformation on the two margins. Whereas the eastern margin is

characterised by a single major fault at the base of the Langano escarpment, the western

margin is marked by several normal faults (including the major Fonko Fault and associated

synthetic and antithetic subparallel structures) distributed over a wider deformation domain

(Figs. 1, 2; Agostini et al., 2011a; Molin and Corti, 2015).

Swath profiles 5 is just south of the boundary between the CMER and SMER, where the E-W

Goba-Bonga lineament (Abbate and Sagri, 1980) is located. Here the Ethiopian Plateau is

characterized by low elevation (~2000 m) and is deeply incised by the Omo River. At the rift

margin, topography decreases progressively towards the rift floor (1500 m). Here the MER

appears to be asymmetrical since to the east, by an elevation drop of ~950 m and a

topography gradient of 0.067 (Table 1), mean topography increases to >2500 m (Somalian

Plateau) and then decreases progressively. Here the Somalian Plateau is strongly incised by

fluvial network. To the west the rift margin presents a small elevation drop (665 m) and

topography gradient (0.019; Table 1); in this portion there is no morphological evidence of a

tectonic escarpment (Fig. 3).

Swath profile 6, across the southernmost portion of the dome-shaped topography, cuts across

the complex region of diffuse deformation including the SMER and the Gofa Province (see

above). The profile reveals the presence of several tectonic basins mostly west of the MER.

Here the MER (corresponding with the Chamo basin) is narrower and symmetrical with both

margins standing at a mean elevation of more than 2000 m and presenting similar elevation

drop and topography gradient values (Table 1). The topography becomes relatively regular on

the southeastern portion (~1500 m in mean elevation) where the profile crosses the

Precambrian rocks standing at the periphery of the dome-shaped topography relative to the

upwelling of the Afar plume (Sembroni et al., 2016a, b and references therein).


3.2.3 Local relief

We constrain a map of local relief of the study area to quantify the difference in elevation

between valley bottoms and peaks, and to investigate the spatial distribution of river incision.

In detail, we smooth the maximum and minimum topography of the SRTM digital elevation

models by a 10 km diameter circular moving window (low-pass filter) in GIS environment.

We choose such a value since it is the average spacing of the main valleys (7th Strahler order

with respect to a critical drainage area of ~4 km2). This allows the removal of small valleys,

highlighting the regional-scale features. The results of the smoothing are two surfaces: the

sub-envelope surface, which describes the general pattern of valley bottoms elevations, and

the envelope surface, that connects peaks elevations. We obtain the local relief map by

arithmetically subtracting the resulting surfaces (Fig. 5a). To demonstrate better the

relationship between topography and river incision, we extract 6 swath profiles across the

local relief map. In more detail, using the same observation windows of Fig. 3, we sample the

local relief along 5 profiles and calculate the mean local relief value. The results are plotted

in Fig. 4 to facilitate the comparison between the trend of local relief and the topography

configuration.

The study area is characterized by a general low local relief (<300 m) with the lowest values

concentrated in the MER and the portions of the Ethiopian and Somalian plateaus preserved

by erosion (Fig. 5a). Such features represent the ancient top surface of the Trap deposits and

have been used by some authors to calculate the volume and thickness of flood basalts (Gani

et al., 2007; Sembroni et al., 2016b) and to reconstruct the post-Trap topography (Sembroni

et al., 2016b). Values >300 m are in the MER where volcanoes and active tectonics cause the

increase of local relief up to 600 m (Figs. 4, 5a). Values >300 m characterize also the inner

sectors of the Ethiopian and Somalian plateaus, where rivers incised deep canyons, and the

rift margins (Figs. 4, 5a). Here the relief increases where the extensional tectonics generates

escarpments and possibly a flexural uplift of the footwall (Sembroni et al., 2016b). Indeed,

relatively high values of local relief are at the footwall of the major border faults, further

evidence for the symmetric or asymmetric structure of the rift (Figs. 4, 5a).

The topography configuration is much more complicated in the southernmost sector of the

MER where several tectonic basins as well as the strong incision of the Omo River make

difficult the reading of the local relief map (Figs. 4, 5a).


3.2.4 ksn map

Since hydrography is sensitive to local and regional tectonic inputs (e.g., Pazzaglia et al.,

1998; Wegmann and Pazzaglia, 2002; Tomkin, et al., 2003; Lock et al., 2006; Wobus et al.,

2006; Scotti et al., 2013; Molin and Corti, 2015; Sembroni et al., 2016a), we investigate the

MER drainage system focusing on the channel steepness variation.

To study river incision both at steady- and transient-state, a power law is widely used which

relates channel slope S and drainage area A (Hack, 1957; Flint, 1974; Howard et al., 1994;

Sklar and Dietrich, 1998; Whipple and Tucker, 1999; Snyder et al., 2000; Whipple et al.,

2000; Schorghofer and Rothman, 2002; Kobor and Roering, 2004):

S = ksA-θ,

(1)

where ks and θ are the steepness and concavity indices. Several studies evidence a positive

correlation between steepness index and uplift rate, although it is strongly influenced by

geologic setting and climate (e.g., Snyder et al., 2000; Kirby and Whipple, 2001; Whipple,

2004; Wobus et al., 2006; Kirby et al., 2007; Molin and Corti, 2015). Under steady-state

conditions, the θ=0.4-0.6, but theoretically ~0.45 (Tarboton et al., 1989; Whipple and Tucker,

1999; Snyder et al., 2000; Kirby and Whipple, 2001; Whipple, 2004; Whipple et al., 2007).

This theoretical value is used as a reference concavity to normalize the steepness index (ksn;

Wobus et al., 2006). This allows the comparison of rivers regardless the size of the drainage

basins. To reveal the general variation in steepness index values along river courses, we

extract the map of the along channel variation in ksn by using the Stream Profiler tool

developed by Whipple et al. (2007); the critical drainage area is set at 107 m2 to obtain a

detailed analysis of the river network up to the 3th Strahler order. Once obtained, the linear

shape file has been converted into a point shape file and interpolated by a nearest neighbour

triangulation algorithm to extract the areal distribution of ksn.

To show the pattern of ksn with respect to topography, we extract 6 swath profiles across the

ksn map. Using the same observation windows as Fig. 3, we sample the ksn map along 5

profiles and calculate the mean value. The results are plotted in Fig. 4 to compare the channel

steepness trend with the topography pattern. Since the channel steepness is very sensitive also

to very small variation in slope, both map and profiles show abrupt changes in values on

short distance.

The ksn map (Fig. 5b) shows a correspondence between high values of ksn (> 350, which

coincide with knickpoints and knickzones) and rock type changes, especially along the


Ethiopian and Somalian plateaus scarps. Medium-high values (>200) correspond to the rift

margins (Figs. 4, 5b) where the tectonic structures have generated knickpoints and

knickzones (Molin and Corti, 2015). At the footwall of the major border faults of MER,

higher values of ksn evidence the symmetric or asymmetric structure of the rift (Figs. 4, 5b).

4. Discussion

We have used structural, topographic and hydrographic analysis to constrain the architecture

and segmentation of the MER, considering the interaction between tectonics and fluvial

erosion and the role of such interaction in dictating the evolution of the rift.

The MER displays significant along-axis structural variation: the MER’s three traditional

subdivisions, the NMER, CMER and SMER (e.g., Agostini et al., 2011b) can be further

subdivided into portions characterized by symmetric or asymmetric structure (Fig. 6a; Table

1). In general, ~70% of the 500 km-long rift is asymmetric. The symmetry or asymmetry of

the rift is reflected in the topography and in the rift margin elevation drop and topography

gradient. The tectonic symmetry/asymmetry is supported by the convergence/divergence of

such values for both rift margins (Table 1). Where major boundary faults are present, the rift

margin reaches higher elevations (Figs. 2, 3, 4), locally showing a decrease at the footwall of

the tectonic structures typical of flexural uplift (Sembroni et al., 2016b). This process

increases elevation and, therefore, stream power and channel steepness (Fig. 5b), that results

in river incision (Fig. 5a). Unloading related to fluvial erosion could have enhanced the

flexural uplift, generating a positive feedback between tectonics and surface processes. This

dynamic coupling between flexural uplift and fluvial incision probably amplified the

symmetric or asymmetric structure of the rift. A similar scarp evolutionary scenario has been

predicted by numerical modeling (Tucker and Slingerland, 1994; Kooi and Beaumont, 1994;

Petit et al., 2007).

Generally, most of the asymmetric sectors have a master fault system located on the eastern

margin. In particular, >70% of the eastern margin is marked by a major boundary fault; the

remainder is marked by minor faulting and monoclines. In some cases (e.g., Asela), the

margin morphology is masked by prominent recent volcanism, and therefore 70% can be

considered a lower bound. Next, we interpret the spatial pattern of border faulting in light of

geophysical constraints on lithospheric structure and strength.


4.1 Relations between rift architecture and pre-rift lithospheric rheology

The MER has developed within a region that has experienced several tectonic events, from

different phases of collision during the Precambrian to Mesozoic extension (e.g., Chorowitz,

2005), which have resulted in significant lateral variations in the rheological structure of the

lithosphere. The rift valley formed within an ancient suture zone sandwiched between two

distinct lithospheric domains beneath the Ethiopian and Somalian plateaus (e.g., Vail, 1983;

Berhe, 1990; Kazmin et al., 1978; Bastow et al., 2008; Cornwell et al, 2010; Keranen and

Klemperer, 2008; see above section 2). Rift architecture and the along-axis structural

variations are likely controlled by differences in the crustal/lithospheric structures of these

two domains.

Specifically, the eastern margin is characterised by limited along-axis variations in

architecture, being dominated by large boundary escarpments along almost its entire length;

this likely reflects the strong and homogeneous nature of the lithosphere beneath the

Somalian Plateau (e.g., Keranen and Klemperer, 2008). Conversely, the western margin is

more irregular, with segments characterised by large boundary faults (Ankober, Fonko-

Guraghe, Gamo-Gidole) alternating with segments marked by gentle flexures towards the rift

axis. This reflects a more complex structure of the Ethiopian plateau lithosphere, interpreted

to comprise two different portions: a strong northern portion, strengthened by cooled (pre-

rift) mafic underplate, and a southern portion, with thinner crust and weaker rheology (Fig.

6b; e.g., Keranen and Klemperer, 2008). This southern portion is marked by the presence of

two major transversal lineaments, YTVL and Goba Bonga, resulting from the Cenozoic

reactivation of pre-existing Neoproterozoic weaknesses, sub-parallel to the trend of the Gulf

of Aden (e.g., Abbate and Sagri, 1980; Abebe et al., 1998; Korme et al., 2004; Abebe

Adhana, 2014). Where these pre-existing weak heterogeneities intersect at a high angle the

rift, the western margin lacks major boundary faults, which are instead replaced by gentle

monoclines; this, together with well-developed faults on the eastern side, results in an overall

asymmetry of the rift (Fig. 6).

Geophysical analysis of the crust and mantle beneath both plateaus (e.g. Dugda et al., 2005;

Whaler et al., 2006; Stuart et al., 2006; Mackenzie et al., 2005; Maguire et al., 2006; Bastow

et al., 2008; Keranen et al., 2009) support this interpretation (Fig. 6). Crustal thickness and

bulk crustal Vp/Vs ratios are relatively heterogeneous beneath the Ethiopian Plateau (41-43

km in the NW, <40 km in the SW) compared to the homogenous crust (38-40 km thickness,

1.8 Vp/Vs ratio) beneath the Somalian Plateau (Fig. 6b; Stuart et al., 2006; Keranen et al.,

2009). The crust beneath the YTVL of the Ethiopian Plateau is characterized by anomalously


high Vp/Vs (1.9-2; e.g., Stuart et al., 2006), which is interpreted as evidence for the presence

of melt in the crust. In support of the interpretation of elevated fluid content in the crust

beneath the YTVL, magnetotelluric data show low resistivities (~2 Ωm), in contrast to the

high resistivities (~100 Ωm) found beneath the Somalian Plateau. Analysis of crustal

thickness indicates that the YTVL corresponds to a sharp gradient in Moho depth (Fig. 6b).

In particular, this transversal lineament corresponds to the boundary of the strong and thick

crust of the northern portion of the Ethiopian Plateau (Keranen and Klemperer, 2008). South

of the YTVL, the Ethiopian Plateau (including the Goba Bonga region) has a crust that is

significantly thinner than the average crustal thickness of the Ethiopian-Somalian plateaus

(see also Keranen et al., 2009). Notably, along-axis variations in crustal thickness in the

Ethiopian Plateau match well the subdivision in symmetric or asymmetric rift portions we

suggest, with major rift escarpments corresponding to portions characterised by thick (and

likely strong) crust (Fig. 6b).

Our interpretation of heterogeneous crustal composition, thickness and strength for the

Ethiopian Plateau is supported by geophysical constraints on properties of the whole plate.

The 75 km depth slice through relative arrival-time body-wave tomographic model of Bastow

et al. (2008) shows distinctive low velocity anomalies that extend from the rift valley to the

Ethiopian Plateau in two lobes specifically beneath the YTVL and Goba Bonga transversal

lineament (Fig. 6b). These low velocity anomalies are best explained by a combination of a

thermal anomaly and partial melting in response to upwelling and decompression of the

asthenosphere caused by localized extension and thinning of the lithosphere beneath the two

volcanic lineaments (e.g., Bastow et al., 2010; Gallacher et al., 2016). In support of the

hypothesis of plate weakening, the values of the effective elastic plate thickness (Te; Pérez-

Gussinyé et al., 2009) are generally much lower and more heterogeneous for the Ethiopian

Plateau than for the Somalian Plateau, where Te can reach values >60 km. The most

prominent asymmetries occur south of Addis Ababa where the Ethiopian Plateau is marked

by major E-W trending YTVL and the Bonga Goba transversal lineament. This latter

lineament corresponds to a narrow E-W domain where Te decreases to 10-15 km (Pérez-

Gussinyé et al., 2009). Whether zones of thinned crust, and inferred thinned lithosphere,

beneath the YTVL and the Bonga Goba transversal lineament are directly caused by pre-rift

lithospheric thin zones, or by syn-rift extension that exploits pre-rift zones of weakness is

impossible to tell. However, both interpretations strongly suggest that pre-rift structural

control exerts a significant influence on the current structure of the lithosphere.


In summary, all the above-mentioned observations suggest that, although different processes

may have contributed to the present architecture of the MER (e.g., flexure during initial

rifting stages, Kazmin et al., 1980; bending related to magma intrusion, Corti et al., 2015;

Buck, 2017), the major control has been likely exerted by the different rheological nature of

the Ethiopian and Somalian plateaus. This supports recent observations from other

continental rifts (e.g., Malawi Rift, Upper Rhine Graben) illustrating that along-axis

variations in basement fabric have a strong influence on basin architecture and segmentation

as well as on the characteristics of the rift margins (e.g., Lao-Davila et al., 2015; Grimmer et

al., 2017).

4.2 General implications for continental rifting and break-up

Overall, our integrated analysis suggests that the MER is characterized by a Mio-Pliocene rift

segmentation controlled by the pre-rift lithospheric structure, which results in 80-100 km rift

portions characterized by alternating symmetric/asymmetric basins. The timescale (~10

Myrs) of the formation of this architecture is at least partially due to the positive feedback

between tectonic and fluvial erosion which emphasizes the flexural uplift of the rift margin

faults and its topographic expression. With increasing extension into the Quaternary, the

along-rift segmentation of the axis is progressively defined by the 40-60 km long magmatic

segments in the NMER (Ebinger and Casey, 2001). This indicates that whereas rift

architecture is mostly controlled by the pre-rift lithospheric structure during initial rifting

stages, other processes (such as magmatism and rift obliquity; e.g., Corti, 2008) define axial

segmentation during more advanced rifting (e.g., Ebinger, 2005).

One important outcome of our work is that there is no evidence for a progressive evolution

from asymmetric to symmetric rifting with time, as proposed by previous works

(WoldeGabriel etal.,1990; Hayward and Ebinger, 1996). Indeed, asymmetric rifts may be

characterized by a direct focusing of the tectonic-magmatic activity at the rift axis, as

observed in the NMER (see section 2 in Fig. 2; Ebinger and Casey, 2001; Ebinger, 2005) and

supported by modelling of rift evolution (e.g., Corti, 2008). An alternative behaviour has

been observed in the northern portion of the SMER, where deformation seems to have

transitioned from an asymmetric structure with master fault on the eastern side to localized

volcano-tectonic activity at the western margin in the Soddo area (Corti et al., 2013), a

process which may have been controlled by localized magma intrusion and a consequent

weakening of the local crust/lithosphere.


Such observations have important implications for the asymmetry of conjugate passive

margins worldwide. Our data suggest that asymmetry may result from the pre-rift asymmetric

nature of the lithosphere, which controls the development of major boundary faults on one

side, and controls the less-developed faulting on the opposite side. The influence of the

rheological and geometrical characteristics of lithosphere on the asymmetry of a rift system

has been evidenced also by numerical models (e.g., Huismans and Beaumont, 2014)

indicating that a strong lithosphere supports large flexural stresses that control the flank uplift

and, consequently, the erosion rate. Therefore, asymmetry does not require low-angle

detachment faults and lithospheric-scale simple shear deformation (e.g., Wernicke, 1985): our

data support the high-angle nature of large boundary faults (e.g., Keir et al., 2006), arguing

against significant simple shear deformation. The topographic expression of these structures

is enhanced by the unloading related to fluvial erosion.

The integrated approach to such a complex context like continental rifting is essential to

understand the role of sub-crustal, crustal and surface processes in generating regional scale

tectonic structures and relative landscape.

5. Conclusions

In this work, along-axis variations in architecture, segmentation and evolution of the different

sectors of the Main Ethiopian Rift (MER), East Africa, have been analysed by means of an

integrated structural and geomorphological analysis and compared with regional geology and

geophysical constraints on lithospheric structure. This analysis suggests the following main

conclusions:

-~70% of the 500 km-long MER is asymmetric, with most asymmetric sectors characterized

by a master fault system on the eastern margin. Thus, >70% of the eastern margin is marked

by a major boundary fault; the rest is marked by minor faulting and monoclines.

-The symmetry or asymmetry of the rift is reflected in the topography and hydrography. The

rift margin topography gradients differ by <40% in the symmetric portions and >100% in the

asymmetric ones. Steep rivers deeply incise the footwall of the major border faults; this

erosional unloading increases the isostatic uplift of footwall, enhancing the

symmetric/asymmetric topography of each sector of the rift.


-Rift architecture and segmentation are strongly controlled by the pre-rift lithospheric

structure, especially the contrasting nature of the homogeneous, strong Somalian Plateau and

the weaker, more heterogeneous Ethiopian Plateau.

-Asymmetric rift sectors may directly progress to focused axial tectonic-magmatic activity,

without transitioning into a symmetric rifting stage.

Acknowledgments

We thank three anonymous reviewers and the Associate Editor Tesfaye Kidane for the

detailed comments which helped to improve this manuscript. We also thank Federico Sani for

discussions. D.K. is supported by NERC grant NE/L013932/1. The data supporting this

paper are listed in the references and available within tables and figures; any further

information can be obtained upon request.


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Table 1. Summary of the characteristics of the different MER sectors (see text for further details)

Sector Length

(km) Structure

Main Border

Fault Active deformation Magmatic activity

Rift margin

elevation drop (m)

Topography gradient

of rift margin

West East West East

1 70 Symmetric -

Mostly axial (minor

activity in the Western

margin)

Axial (Wonji) 1799 1750 0.047

0.040

2 100 Asymmetric East Axial Axial (Wonji) 1500 1416 0.019 0.042

3 50 Asymmetric West Marginal Marginal

(Wonji+SDZ) 1392 870 0.058 0.029

4 60 Symmetric - Mostly marginal (axial

subordinate) Axial (WFB) 946 897 0.019 0.027

5 100 Asymmetric East Marginal (activity at the

Western margin)

Marginal (mixed

Wonji+SDZ

characteristics)

665 948 0.019 0.067

6 120

Symmetric

(Chamo basin)

Asymmetric

(Galana basin,

Gofa Province)

East and

West

Mostly marginal (axial

subordinate) Almost absent

1164.8

(Chamo

basin)

1248.2

(Chamo

basin)

0.042

(Chamo

basin)

0.037

(Chamo

basin)


Figure 1. Fault pattern and Quaternary volcanics in the Main Ethiopian Rift (MER)

superimposed on a NASA's Shuttle Radar Topography Mission (SRTM) digital elevation

model. The three main rift sectors are labelled as: NMER, Northern MER; CMER, Central

MER; SMER, Southern MER. Dashed lines with numbers indicate the cross-sections

illustrated in Fig. 2. Volcanic fields and calderas are labelled as follows: Al: Aluto; Bo,

Boseti; Co, Corbetti; Do, Dofan; Du, Duguna; DZ, Debre Zeyt; Fa, Fantale; Ge, Gedemsa;

Ko, Kone; NA, North Abaya; SB, Silti-Butajira; TM, Tulu Moye; Zw, Zikwala. Lakes

labelled as follows: Ab, Lake Abijata; Ay, Abaya; Aw, Awasa; C, Lake Chamo; Ch: Chilalo;


K, Lake Koka; Ln, Langano; Sh, Shala; Zw, Lake Ziway. Other labels: GBL: Goba-Bonga

transversal lineament; So: Soddo; YTVL: Yerer-Tullu Wellel volcano-tectonic lineament.


Figure 2. Simplified geological profiles across different sectors of the MER. Topographic

profiles are extracted from SRTM digital elevation models, with vertical exaggeration of 10.


WFB: Wonji Fault Belt; SDZ: Silti Debre Zeit volcanic belt. Sources for the geological

profiles: Abebe et al. (2005); Agostini et al. (2011a); Corti et al. (2013); Davidson (1983);

Ebinger et al. (1993, 2000); Ethiopian Mapping Agency (1978, 1981); Ethiopian Mapping

Authority (1996); Hutchison et al. (2015); Philippon et al. (2014); Wolfenden et al. (2004).


Figure 3. Topography of the study area (SRTM digital elevation model, 90 m of resolution)

including the main tectonic structures (solid and dashed lines) and the swath profiles location.

Ch: Mt Chilalo; Ck: Mt Chilke; Ka: Mt Kaka.


Figure 4. Swath profiles illustrating the maximum, mean, and minimum elevation pattern

cutting across the MER. The plots include also the variation in the mean values of local relief

and channel steepness index (see text for explanation and Fig. 5 for profiles location).


Figure 5. a) Map of the local relief of the study area: note how the correspondence between

high values of local relief at the rift margins and location of the main structures evidences the


symmetry/asymmetry of the MER sectors. b) Map of the along channel variation of the

steepness index: along the rift margins high values of channel steepness correspond with the

main tectonic structures.


Figure 6. a) Interpreted along-axis variations in the structure of the Main Ethiopian Rift.

Single arrows indicate the polarity of the different rift sectors; double arrows indicate


symmetry of rifting. Whitish boxes indicate the hypothesised extent of the transversal YTVL:

Yerer-Tullu Wellel (YTWL) and Goba-Bonga (GBL) volcano-tectonic lineaments. b)

Contour map of Moho depth in the MER (after Keranen et al., 2009). c) Depth slices through

the P wave velocity model of Bastow et al. (2008) at 75 km depth. See text for further details.

control of pre-rift lithospheric structure on the ...continental rifts and conjugate passive margins...

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